Many people think that in the thousands of years following the rise of agriculture, human societies were static. They were not. Empires rose—some flourished, then perished, while others persisted. Most people remained subsistence farmers who kept themselves, or themselves and the ruling elites, alive. Foraging as a way of life was pushed to agriculturally marginal lands. Populations grew rapidly, with estimates ranging from between 1 and 10 million people at the beginning of agriculture to between 425 and 540 million in the year 1500, around 10,000 years later.

In the 16th century, everything began to change, and change with increasing speed. Agricultural development, from simpler farming communities to city-state to empire (and often back again), slowly began to be replaced by a new mode of living. Revolutions in what people ate, how they communicated, what they thought, and their relationship with the land that nourished them emerged. Somehow, those living on the western edge of the continent of Europe changed the trajectory of the development of human society, and changed the trajectory of the development of the Earth system, creating the modern world we live in today. Nothing would be the same again.

A pivotal moment in this shift to the modern world was the arrival of Europeans in what they would name America. The people of the Americas had been isolated from those of Asia and Europe for about 12,000 years, aside from the odd visit from a lost Viking ship to the North American Atlantic shoreline and rare Polynesian forays to the South American Pacific Coast. This separation of humanity occurred because at the end of the last ice age, as the world warmed, there was still enough ice for a few individuals to make it across the Bering Strait from Asia to North America. This window of opportunity to cross did not last long, as most of the sea ice melted, closing the route. The few who made the Bering Strait crossing spread out across the Americas and slowly populated the entire land mass.

After 12,000 years of separation, Native Americans met Europeans on unequal terms. Almost all the major species of domesticated livestock were from Eurasia, and the livestock that tend to live closest to humans (cow, sheep, goat, pig, and horse) had been living with Europeans for thousands of years. These provided plentiful opportunities for diseases to pass from animal to human and vice versa, and to spread across Eurasia, from eastern China to western Spain. When Christopher Columbus arrived in the Caribbean for the second time, in 1493, he planned to settle. He arrived with 17 ships, 1,500 people, and hundreds of pigs and other animals. As soon as they landed on December 8, the pigs, which had been isolated in the very bottom of the boat, were released.

The next day, the Europeans began to fall ill, Columbus included. Native Americans began to die. This was probably swine flu, to which Native Americans had no prior exposure. Twenty-three years later, in 1516, the Spanish historian Bartolomé de las Casas wrote of the island that is now Haiti and the Dominican Republic: “Hispaniola is depopulated, robbed and destroyed … because in just four months, one-third of the Indians [the Spaniards] had in their care have died.” Two years later in Memorial on Remedies for the Indies, he wrote that “of the 1,000,000 souls there were in Hispaniola, the Christians have left but 8,000 or 9,000, the rest have died.” But worse was to come.

Long voyages from Europe originally worked as a type of quarantine for passengers with smallpox, as it is only infectious for up to a month. Carriers either died on the ship or arrived with added immunity. Either way, smallpox did not survive the journey. As better ships with improved sails cut the crossing time, new diseases could hitch a ride. Smallpox arrived on Hispaniola by January 1519, and spread immediately to the mainland of Central America. Native Americans had no immunity to smallpox, influenza, or the other diseases brought from Europe. These infections hastened the Spanish conquest of what is commonly known as the Aztec Empire—a term invented in the 19th century—or more correctly the Mexican Triple Alliance, after the 1428 treaty between the rulers of three cities.

As Spaniards pillaged, their diseases helped them. In August 1519 when Hernán Cortés had initially attempted to take the largest city in pre–Columbian America, the 200,000-strong Mēxihco-Tenōchtitlan, he narrowly escaped with his life. But as he regrouped, disease ravaged Tenōchtitlan. After a 75-day siege, deaths from disease, combat, and starvation had left one of the largest cities in the world almost lifeless. With a few hundred Spaniards and the Tlaxcalans, rivals to Mēxihco-Tenōchtitlan, on August 13, 1521, Cortés claimed Tenōchtitlan for Spain.

One of Cortés’ solders, Bernal Díaz del Castillo, wrote, “I swear that all the houses on the lake were full of heads and corpses … The streets, squares, houses, and courts were filled with bodies so that it was almost impossible to pass.” Native Americans fought on, but they could not overcome wave after wave of disease, resulting food shortages, and superior Spanish warfare technology. So ended a fast-expanding empire that was the same size as modern-day Italy, 300,000 square kilometers, and whose population numbered somewhere between 11 and 25 million people. Only about 2 million survived the conquest.

The new diseases spread down through Panama, with a contemporary visiting historian estimating that more than 2 million died there from 1514 to 1530. From there the march of infective agents then continued through the Darien Gap and into South America. The largest empire in the Americas—and by some measures the largest in the world at that time—was that of the Incas, whose lands stretched along the backbone of the continent, the Andes mountains. Francisco Pizarro, another Spanish conquistador, made contact with the Incas in 1526, without invading. Some estimate that it only took a year after the meeting for Huayna Capac to become the first Inca ruler to die in the epidemic.

Unlike the Tenōchtitlan catastrophe, the unfolding end of the Inca Empire is more difficult to piece together, because writing was not part of the Inca civilization, and the Spanish only heard of Capac’s death in 1531. Many say he died of smallpox, but a careful reading of the various accounts, including descriptions of the mummified body, suggests he more likely succumbed to one of the more easily transmitted and faster-spreading European diseases, such as measles or influenza. Regardless, the Incas were fatally weakened, and their empire, of 2 million square kilometers and an estimated 10 to 25 million people, was overrun by Pizarro’s men. The Incas did, it seems, keep population records using a system of knots on string called quipi, but the knowledge of how to decipher these was lost as four centuries of rapidly evolving Inca civilization was destroyed. Again, exact numbers are not known, but researchers estimate that about half the population died at the time of immediate conquest.

When trying to understand the catastrophic loss of Native American life, many mistakenly focus only on smallpox. This was an important killer, but by no means the only one. Influenza, measles, typhus, pneumonia, scarlet fever, malaria, and yellow fever, amongst others, arrived in wave after wave. Added to this were the casualties of the wars against the Spanish and later the Portuguese, English, and French, plus those worked to death after being forced into slavery. Such was the chaos of the changes and the loss of so many lives, traditional societies were largely destroyed and farming collapsed—and so famine added to the death toll. It appears that at least 70 percent of people died following sustained European contact, and often 90 percent or more, according to information from the better-studied villages, towns, and regions.

Did this rejoining of two branches of humanity after 12,000 years of separation change Earth’s history as well as human history? The global mixing of humans and their deadly diseases is just one aspect of a much larger global biological mixing that the historian Alfred Crosby called the Columbian Exchange. Not only did pathogens travel, so did plants and animals. Species moved from one continent to another, and one ocean basin to another, outside their evolutionary context. This led to a globalization and homogenization of the world’s species, which continues today.

Most dramatically, the Columbian Exchange transformed farming and human diets. This change is often so culturally ingrained that we take it for granted. It is difficult to conceive that in Europe there were no potatoes or tomatoes before the 16th century; in the Americas, no wheat or bananas; no chili peppers in China or India; and no peanuts in Africa. The transformation of diets was near total: even deep in the Congo rainforest, the staple is cassava, a plant originally from South America, while deep in the Amazon rainforest the Yanomami eat plantains, which were domesticated in Africa.

Farmers, from the 16th century onward, suddenly had a much greater number of crops and animals to choose from. The best crop for the local environmental conditions, sourced from anywhere in the world, could now be planted. People picked the ones that worked well, incorporating them into new farming systems. The increase in the diversity of crops planted in any one place was also a boon to farmers worldwide. These new crops not only improved yields. In China, for example, the arrival of maize allowed drier lands to be farmed, driving new waves of deforestation and a large population increase.

Despite the transport of new killer diseases, including the emergence of deadly syphilis in Europe and Asia, which was linked to trade with the Americas, the Columbian Exchange eventually allowed more people to live off the land. These newly available plants and animals led to the single largest improvement in farm productivity since the original agricultural revolution. The results of different peoples’ efforts in domesticating and refining crops over thousands of years were now available and being adopted worldwide. A single globalized farming culture was born.

In geological terms, transcontinental shipping, which began in the 16th century, and later aviation, which took off in the 20th century, are playing the same role as plate tectonics has in the past. Today, they are knitting the continents and oceans together, the opposite of the trend over the past 200 million years that has seen the continents separating. When geologists inspect the geological record millions of years in the future, fossilized species will be recorded as instantaneously arriving on new continents and in new ocean basins. These fossilized species that humans have allowed to jump geographical barriers will give the appearance of a new species having evolved, just like in other epochs in Earth’s history. But there will also be a subtly different pattern. Normally in the geological record there are extinctions, which in turn create vacant niches, which evolution fills with new, often quite different-looking species. In the human epoch, the sudden appearance of species that have jumped continents, or new hybrid species, will appear in the geological record as being quite similar to already existing species. This homogenization of Earth’s biological diversity is one key hallmark of the Anthropocene, with no obvious past analogue in Earth’s history.

These changes to life are of geological importance. Two hundred million years ago, all of Earth’s land was linked together in the supercontinent of Pangaea, which then broke into separate pieces, with these new continents slowly moving to the positions on the Earth that we are familiar with today. The genetic material left on each separating continent has been evolving largely independently ever since. Transcontinental shipping began to link the continents back together, both deliberately, as people moved selected species, and inadvertently, as stowaway species smuggled themselves to new lands. In the 16th century a new, planet-wide, human-driven evolutionary experiment began which will continue to play out indefinitely. What plate tectonics did over tens of millions of years is being undone by shipping in a few centuries and aviation in a few decades. We are creating a new Pangaea. This fits one of the hallmarks of a new epoch, as it is a geologically significant change to life on Earth. It is an important event in the context of Earth’s history.